In most cases, fears among parents over vaccines -- many of them unfounded -- may be at play.
"The CDC and health departments are doing a good job of increasing vaccine coverage," Omer said. "Therefore, rates of vaccine-preventable disease are going down substantially. Parents aren't seeing the actual diseases, so when they hear about real or perceived adverse effects of vaccines, their perception of the risks versus benefits is shifted."
Dr. William Schaffner, chair of preventive medicine at Vanderbilt University, agreed.
"Parents are just more skeptical about benefits of vaccines," he said. "Most young parents today have never seen a case of polio or measles, and they didn't learn about the seriousness of these diseases and importance of the vaccinations in school."
Schaffner added that he finds it interesting that some states do not allow exemptions while others "really oblige parents wishing to opt out."
Past research has shown, though, that in states with a substantial proportion of unimmunized or incompletely immunized children, many kids are susceptible to these classic diseases. A 2006 study showed a 50 percent higher incidence of pertussis -- commonly known as whooping cough -- in states where it is easier to get exemptions. There have been similar findings with respect to measles.
Schaffner said he is very concerned that unvaccinated children who go abroad will bring back diseases, such as measles, that are still a major problem in other parts of the world. Not only will they suffer, but they will spread the illnesses to children who are unable to be vaccinated for medical reasons.
Schaffner said that part of the solution to the problem is that "states with easier process need to tighten up." However, he said, this is not a fool-proof approach. He said some research has shown an increase in medical exemptions in states that have tightened up their policies -- suggesting that parents are pressuring doctors to give medical exemptions. He encourages doctor to not let themselves be "brow-beaten into providing dubious excuses."